1. Field of the Invention
This invention relates to a real image mode finder optical system and, in particular, to a real image mode finder optical system used in cameras such as camcorders and still cameras.
2. Description of the Related Art
A real image mode finder optical system of prior art, which allows the angle adjustment of an eyepiece section, is set forth, for example, in Japanese Patent Preliminary Publication No. Hei 3-81749 as shown in FIG. 1. This is a Keplerian real image mode finder optical system of the primary imaging type, in which a prism 4 with a roof surface 3 is disposed between an objective lens 1 and a field lens 2. A mirror 6 which can be rotated about a rotating axis 5 is situated downward behind the prism 4. An eyepiece 8 is movably disposed to have an optical axis 7a or 7b in accordance with a change in direction of a ray, which has been incident on the objective lens 1 to travel in a path coinciding with the optical axis 7c of the objective lens 1, by the mirror 6.
A beam of light incident on the objective lens is introduced in the prism 4. The light beam, after being totally reflected by a surface for reflecting light 9 (hereinafter referred to as a reflecting surface) in the prism 4, is reflected further by the roof surface 3 and emerges downward from the prism 4. In the vicinity of the exit surface of the prism 4, an external image, not shown, is formed, and the field lens 2 disposed adjacent to this primary imaging plane leads the light beam to the rotatable mirror 6. When the mirror 6 is placed at the position indicated by solid lines in FIG. 1, the light beam represented by the ray reflected from the mirror 6 to travel in the path coinciding with the optical axis 7a is nearly parallel to the optical axis 7c of the objective lens 1 and is introduced to the eyepiece 8 to form an exit pupil 10a behind it. As indicated by arrows in FIG. 1, the rotatable mirror 6 is turned to the position indicated by broken lines, and the eyepiece 8 is moved to the position also indicated by broken lines to have the optical axis 7b according as the direction of the light beam is changed by the mirror 6. As a result, the light beam incident on the mirror 6 is reflected and introduced to the eyepiece 8 deviated obliquely upward to form an exit pupil 10b behind it.
An observer, when adjusting the pupil of his eye to the exit pupil, can observe the external image. Further, by adjusting the mirror 6 and the eyepiece 8, he can change the optical axis of the eyepiece continuously from one horizontally directed to another obliquely directed upward (and such a change is hereinafter referred to as a variable angle).
In the conventional real image mode finder optical system of the primary imaging type, however, the relationship between the focal length of the objective lens and the field angle makes it impossible to provide the variable angle ranging from the horizontal direction to the vertical direction, namely from an eye level to a waist level, with a proper field angle maintained.
The reason for this is explained below. The variable angle needs a plurality of reflecting surfaces between the lenses as illustrated by the prior art. In order to enable the variable angle over a wide range in particular, it is necessary to provide sufficient space between the lenses in view of the case where any of these reflecting surfaces is rotated. Consequently, the focal length of the objective lens must be increased. This causes the increase of the image height of the primary imaging plane, and thus indicates the necessity of enlarging the prism 4, followed by the field lens 2, the mirror 6, and the eyepiece 8. With such an arrangement, however, not only is the weight of the camera increased, but also part of the light beam is eclipsed by this large-sized optical system in accordance with the change in the observation angle. As a result, the range of the variable angle will not be extended if, on the other hand, the field lens 2 and the eyepiece 8 are made smaller in size, they can receive only a limited portion of a light beam of a large field angle will and the external image of a proper field angle will not be obtained. In other words, if the focal length of the objective lens is increased without enlargement of the field lens or the like, the field angle of an image which is necessarily obtained will be limited to a smaller one.
As mentioned above, the conventional real image mode finder optical system of the primary imaging type cannot provide the variable angle ranging from the eye level to the waist level with a proper field angle maintained, and thus is restricted in an observer's posture of observation and the like. Although Japanese Patent Preliminary Publication Nos. Sho 47-33634 and Sho 48-31940 disclose real image mode finder optical systems, each of which has the variable angle ranging from 0.degree. to 90.degree., they do not in any way teach arrangements for securing a wide field angle. Also, the application of an electronic finder using an LCD (liquid crystal display panel) is unfavorable because it is higher in cost than an optical finder.